The present invention relates to motor protection systems and more particularly relates to a system for protecting a multi-phase alternating current (AC) motor from current overload and current imbalance.
Motor protection systems are known for preventing damage to electrical motors due to excessive current. Some of these systems include circuitry for sensing the current applied to a three-phase alternating current (AC) electrical motor and for terminating the supply of current to the motor when the sensed current rises above a predetermined level. Conventionally, these systems utilize a three-phase diode bridge to convert a sensed AC signal into a rectified direct current (DC) output. Inherently, the use of the diodes limits the sensitivity of the circuitry because of voltage losses associated with the operation of diodes. In addition, it is difficult to detect current imbalance with these systems because the DC output is a composite waveform corresponding to all of the three AC motor current phases. Thus, if the magnitude of one phase of the current is very high while the magnitude of a different phase of the current is very low, such that the magnitude of the combined current phases is normal, these systems detect no problem even though the magnitude of the one current phase is excessively high.
The advent of microprocessors and microcomputer systems has provided an opportunity for improving the performance of motor protection systems. A microcomputer system can be used to process a voltage signal from a motor protection circuit to obtain information about the current flow through each motor phase. Conventional motor protection systems are capable of being used with such microcomputer systems. However, the voltage signals generated by the circuitry of these systems require large amounts of computer memory to properly process the voltage signals and information regarding individual motor phases is difficult if not impossible to obtain from these signals. Thus, the capabilities of a microcomputer are not efficiently, and cost-effectively utilized if voltage signals from these conventional motor protection circuits are used by a microcomputer to provide a motor protection system.